In some designs for nuclear power plants, water is heated to form superheated steam which is used to drive turbines which in turn provide the mechanical energy necessary to generate electricity. The water and steam are often flowed through helically coiled tubes which are heated by helium which has been used to cool a nuclear reactor. Helically coiled tubes have been used primarily because of their economy. The helical coils provide for a fairly large heat transfer surface within a relatively small space. Helium heated helical coil heat exchangers have have certain disadvantages. For one, it is difficult to inspect the walls of the tubes tube for defects after they have been in service. This is so because an inspection device cannot be easily passed through a coiled tube. Another disadvantage is due to the fact that if helical tubes are to be used in a heat exchanger where there are economizer, evaporator and superheater tubes, more than one tube sheet will be necessary in order to protect against large temperature gradients created by the difference in temperatures of the fluid entering and leaving the helical coils.
The disadvantages can be overcome to some extent by utilizing bayonet tubes in place of the helical coil tubes. Bayonet tubes can be inspected much more easily than helical coil tubes because inspection devices can be inserted and withdrawn quite easily. However, a heat exchanger using bayonet tubes will retain many of the dissadvantages found in a heat exchanger in which helical tubes are employed. For one, if tubes used for different phases of steam generation such as economizer tubes, evaporator tubes, and superheater tubes are connected to the same tube sheet, a large temperature gradient will result because of the different temperatures of the fluids flowing through the bayonet tubes and which exchange heat with the tube sheet. The use of several tube sheets makes for a design which requires a comparatively large amount of space and which is expensive to build.